In the age of digital imaging and high-quality image-editing software, the maxim “Seeing is believing” is becoming increasingly out-dated. Nowadays, few cosmetic or clothing advertisements, billboards, magazine photos of celebrities, etc., are not digitally altered in some way. In fact, so many people have come to internalize a visually false portrayal of “reality” that at its 2011 meeting, the American Medical Association adopted a proposal calling on advertisers to develop guidelines to discourage the altering of photographs in a manner that could promote unrealistic expectations of appropriate body image. One attendee observed: “The appearance of advertisements with extremely altered models can create unrealistic expectations of appropriate body image. In one image, a model's waist was slimmed so severely, her head appeared to be wider than her waist.” The issue of altered images warping peoples' perceptions of some form of ideal has even reached the political level. In 2009, for example, various politicians in France proposed a law that would require a “warning” label on all advertising, press, political campaign, art photography and packaging images if an image has been retouched, in particular, digitally altered.
Alteration of image content is only one aspect of the concerns involving image manipulation: In many cases, the time an image is created may also be essential information. For example, the time an image was first created can be the determining factor in the relevance of photographic or video evidence in a criminal trial. At present, in many jurisdictions, whether an image or sound recording is admissible as trial evidence is typically left to the discretion of the judge, and whether the admitted evidence proves the occurrence or non-occurrence of some event at a given time is often a question that the court—either the judge or jury or both—decides at least in part subjectively. This means that the accuracy of the determination is a function of the relative sophistication of the forger versus the court.
Similarly, a casino manager or a punter watching an online horse race will want to know if the video he is watching is really “live”, or at least is showing events at the times they are supposed to have happened. With known technology, these viewers must mostly simply trust that there is no manipulation happening, hoping that later information confirms what they saw/heard.
As every tourist knows, a camera can be set to show the time/date a photo was taken. This time/date value, however, proves little or nothing, since it is so easy to change these settings in a camera. The time/date value could be derived from some external source, but this then simply moves the question of reliability from the local device to that external source and the transmission medium between the two.
A more sophisticated method would be to submit the contents of the image to a service that digitally time-stamps them, perhaps along with image metadata. One of the problems with traditional digital time-stamping, however, is that it proves only that data existed before a particular point in time—one can easily take a photo, edit it, then later digitally time-stamp the edited version, thereby forward-dating the image. In other words, typical time-stamping can establish the latest time an image could have been time-stamped, but this doesn't prove that it couldn't have been created and altered earlier. Traditional digital time-stamping works well if there is general acceptance of the time something happened. For example, if a major unexpected news event is generally known to have occurred at 11:37:46 UTC and a photo of the event is digitally time-stamped indicating 11:37:46 UTC, then there is an exceptionally low probability that the photographer will have had time to edit the photograph at all before obtaining the time stamp. Absent such an external confirmatory event, however, conventional digital time-stamping may provide a high level of assurance that an image hasn't been back-dated, but it typically cannot enable detection of forward-dating.
One other drawback of traditional digital time-stamping schemes is the nature of the schemes themselves. Many known digital time stamps rely on a public key infrastructure (PKI). In the context of time determination, one disadvantage of PKI-based time-stamping systems is that users of such systems must simply trust the accuracy of the system's time reading, even though there is no ability to independently verify it after the fact. One other disadvantage of such PKI-based signing schemes is that, by their very nature, they require the creation, administration and maintenance of the keys. Moreover, for reasons of security, digital certificates, and the PKI keys underlying them, are often allowed to expire. PKI keys also have an operational lifetime after which information that has been time-stamped with those keys needs to be re-time-stamped in order to ensure the time stamp is still valid. A compromise of those keys, or even the possibility of a compromise (by insiders intent on fraud or outsiders, such as hackers, intent on profiting from a key compromise) will cause the digital time stamp to be easily challenged. Digital time-stamping is thus not absolute proof but rather more an attestation by the authority that administers the keys.
The problems just described also apply to other types of data. For example, there may be a need to verify the time of a recorded audio event. Audio files are often even easier to edit and forward-date than video.
Part of the problem of temporal visual/audible data verification is that conventional time is deterministic and therefore predictable: If one knows the exact time right now, then one will also know the exact time n seconds (or other time unit) from now. This means that one has n seconds from actual occurrence/creation of an event to manipulate the corresponding data file and then have the altered data stamped with a desired future time.
It would therefore be good to have a way to establish the time of a perceptibly created or recorded event with less opportunity for undetected manipulation. More generally, it would be advantageous to have some representation of time that isn't deterministic and therefore predictable.